As is known, radar and communication systems often employ signal processing arrangements which require very narrow-band filters with low-transient responses. Crystal filters, because of their low insertion loss, good temperature stability and small size are ideal for these applications; however certain classes of responses have not been realizable in this manner. Bandpass crystal filters are usually divided into three classes, narrow-band having bandwidths less than 0.2%; intermediate-band having bandwidths between 0.2% and 1%; and wide-band having bandwidths greater than 1%. No practical realization has heretofore existed for intermediate-band crystal filters with low-transient responses.
It is known that an intermediate-band crystal filter response is realizable if it can be simulated by a lattice network or a cascade of lattice networks containing only series resonant circuits to simulate crystal resonances. Intermediate-band filters derived from symmetric or anti-metric low-pass networks can always be realized as a complicated lattice network. Thus, the prototype low-pass ladder network can first be transformed into a lattice network and then transformed to a band-pass crystal filter. Unfortunately, the low-transient prototype networks are neither symmetric nor anti-metric, accounting for the absence of any previous teaching of techniques for realizing intermediate-band, low-transient filters.
At the present time, low-transient responses are achieved by wide-band crystal filters, helical filters or possibly LC filters. However, these approaches have several disadvantages. First, wide-band crystal filters incorporate inductances as an integral resonating component of the filter rather than a transforming device. Consequently, the inductance Q requirement is high and usually necessitates ferrite as a core material. The filter response does not remain within the required limits over the military temperature range of -55.degree. C to +95.degree. C because it is very difficult to find a ferrite material possessing suitable linear temperature characteristics over this wide temperature range. Thus, suitable compensation with polystyrene capacitors is impossible over the entire range. If powdered iron is used as the core material, the necessary Q is obtained at the expense of increased size. In addition, temperature compensation is then accomplished only by "hand-picking" the temperature compensating capacitors. Furthermore, the element values become impractical at the lower end of the intermediate band.
A second disadvantage resides in the fact that for narrow bandwidths, helical filters are relatively large; their insertion loss is appreciable; and, moreover, the insertion loss varies with temperature. This latter feature is extremely undesirable and compensation is employed to remedy the situation. For such applications, therefore, helical filters are only a marginal solution.
LC filters are rarely used to achieve intermediate-band low-transient responses and then only for the wider bandwidths (e.g., 1%). Again, the Q requirement is prohibitive and the insertion loss is too high. This, in turn, increases the filter's size.